1. Field of the Invention
The present invention relates to a gravity-compensation type accelerometer. Compensating for the effect of gravity on the seismic mass of an accelerometer gives increased sensitivity to variations in acceleration.
The invention applies particularly to small apparatuses. The nature of the accelerometer makes it suitable for construction using mechanical, micromechanical or microelectronics techniques (for example micromachining).
The main field of application for an accelerometer using the present invention is for studying the movement or behavior of milieus subject to gravity (for example seismology).
2. Discussion of the Background
The invention thus makes it possible to design gravity-compensation accelerometers where known types of one-piece accelerometer do not allow for this compensation. Accelerometers using known techniques are described in, for example, M. Ueda, H. Inada, Y. Mine and K. Sunago: "Development of micromachined silicon accelerometer" in the Sumitomo Electric Technical Review, No. 38 of June 1994, pages 72-77 and in Michael E. Hoenk: "Small inertial measurements units-sources of error and limitations on accuracy" in the review SPIE, vol. 2220, pages 15-26.
The most common method for measuring acceleration consists in not directly measuring the acceleration itself but rather the force F applied to a mass M due to the effect of the acceleration .gamma. in question. According to the basic law of motion F=M..gamma., if the value of M is known, F can be measured and a value for the acceleration obtained.
The most common type of acceleration sensor thus consists of an inert, or seismic, mass, generally supported by one or more springs. When the mass is subjected to variations in acceleration, it moves and the springs are distorted. The system returns to its initial position as soon as the force due to the acceleration is canceled.
A horizontal acceleration sensor in the rest state is not sensitive to any disruptive effect. On the other hand, a vertically-sensitive accelerometer is subject to a minimum force equivalent to that of gravity, F=M.g, where g is the gravitational constant.
This minimum force due to gravity is inconvenient when attempting to measure very slight vertical accelerations (less than 10.sup.-6 G). It is therefore important in this situation to compensate for the effort due to gravity with a force tending in the opposite direction to that exerted by gravity. At the present time there are two classes of processes for compensating for the force of gravity:
processes using a source of electric power. An electromagnetic or electrostatic field maintains the seismic mass in suspension. Such processes require complex servo systems, PA1 processes using the return force of a spring. The mass is maintained in a state of equilibrium, suspended by a pre-distorted spring. PA1 .omega..sub..gamma. =resonance pulse PA1 k.sub.b =Boltzman constant PA1 T=temperature PA1 M=mass of seismic mass PA1 masking of one of the main surfaces of a substrate, the first surface, to delimit the seismic mass and the support, PA1 engraving the first surface of the substrate in the direction of the other main surface of the substrate, the second surface, leaving a membrane between the bottom of the engraving and the second surface, this engraving delimiting the seismic mass and the support, PA1 masking the second face of the substrate in such a way as to mask the support, the seismic mass and the mechanical connecting means, PA1 engraving the second surface of the substrate to open unmasked areas of the membrane, PA1 processing at least part of the surface of the mechanical connecting means to induce a prestress counteracting the force exerted on the seismic mass by gravity, thereby constituting said means of compensation.
There are also hybrid systems that use a combination of electrostatic or electromagnetic forces together with the return force of a spring. An example of this type of system is described in Shi Jung Chen and Kuan Chen: "The effects of spring and magnetic distortions on electromagnetic geophones" in J. Phys. E. Sci. Instrum. 21 (1988), pages 943-947.
These techniques have other disadvantages.
In electrostatic or electromagnetic apparatuses, the presence of an electronic servo system can generate interference noise that is incompatible with the desired sensitivity. Moreover, purely electrostatic compensation methods produce unstable systems that are difficult to servo-control.
Vertically-sensitive sensors where the effect of gravity on the seismic mass is compensated for by a spring are currently produced by assembling a variety of mechanical parts. This type of sensor is described in, for example, E. Wielandt and G. Streckeisen: "The leaf-spring seismometer: design and performance" in Bulletin of Seismological Society of America, Vol. 72 No. 6; pages 2349-2367, December 1982. By virtue of their construction, this type of apparatus does not have a very high Q quality factor. This structural parameter is related to the density of Brownian noise S of the apparatus using the following relation: ##EQU1## where: .omega.=pulse
For more information on this relation, please refer to the article "Small inertial measurements units-sources of error and limitations on accuracy" cited above. The relation shows that S is inversely proportional to Q and M.
To preserve levels of Brownian noise that do not disrupt measurement, present apparatuses have a significant mass M. However, this solution limits miniaturization of the assembly. Of accelerometers with the best performance characteristics (for example, those capable of detecting variations of a few nano-G below 1 G), the smallest weigh several kilograms and have volumes measured in tens of cubic centimeters.
In conclusion, vertically-sensitive accelerometers are either very insensitive or are heavy and bulky. Miniaturization of a high-performance device would require a reduction in size of the seismic mass to increase the quality factor. This may be achieved by using a material with a high quality factor such as, for example, mono-crystalline silicon for manufacturing the sensor assembly. However, building a compact apparatus that includes a spring fastened to the seismic mass presents some technical difficulties. In practice it is difficult to fasten small mechanical parts such as the spring and the seismic mass to one another using mechanical means such as screws or cement without creating areas where internal friction is significant, causing damping phenomena prejudicial to the quality factor. Moreover, the spring must retain a high degree of flexibility since this flexibility influences the sensitivity of the sensor.